mcheckmark.go

Documentation: runtime

		 1  // Copyright 2020 The Go Authors. All rights reserved.
		 2  // Use of this source code is governed by a BSD-style
		 3  // license that can be found in the LICENSE file.
		 4  
		 5  // GC checkmarks
		 6  //
		 7  // In a concurrent garbage collector, one worries about failing to mark
		 8  // a live object due to mutations without write barriers or bugs in the
		 9  // collector implementation. As a sanity check, the GC has a 'checkmark'
		10  // mode that retraverses the object graph with the world stopped, to make
		11  // sure that everything that should be marked is marked.
		12  
		13  package runtime
		14  
		15  import (
		16  	"runtime/internal/atomic"
		17  	"runtime/internal/sys"
		18  	"unsafe"
		19  )
		20  
		21  // A checkmarksMap stores the GC marks in "checkmarks" mode. It is a
		22  // per-arena bitmap with a bit for every word in the arena. The mark
		23  // is stored on the bit corresponding to the first word of the marked
		24  // allocation.
		25  //
		26  //go:notinheap
		27  type checkmarksMap [heapArenaBytes / sys.PtrSize / 8]uint8
		28  
		29  // If useCheckmark is true, marking of an object uses the checkmark
		30  // bits instead of the standard mark bits.
		31  var useCheckmark = false
		32  
		33  // startCheckmarks prepares for the checkmarks phase.
		34  //
		35  // The world must be stopped.
		36  func startCheckmarks() {
		37  	assertWorldStopped()
		38  
		39  	// Clear all checkmarks.
		40  	for _, ai := range mheap_.allArenas {
		41  		arena := mheap_.arenas[ai.l1()][ai.l2()]
		42  		bitmap := arena.checkmarks
		43  
		44  		if bitmap == nil {
		45  			// Allocate bitmap on first use.
		46  			bitmap = (*checkmarksMap)(persistentalloc(unsafe.Sizeof(*bitmap), 0, &memstats.gcMiscSys))
		47  			if bitmap == nil {
		48  				throw("out of memory allocating checkmarks bitmap")
		49  			}
		50  			arena.checkmarks = bitmap
		51  		} else {
		52  			// Otherwise clear the existing bitmap.
		53  			for i := range bitmap {
		54  				bitmap[i] = 0
		55  			}
		56  		}
		57  	}
		58  	// Enable checkmarking.
		59  	useCheckmark = true
		60  }
		61  
		62  // endCheckmarks ends the checkmarks phase.
		63  func endCheckmarks() {
		64  	if gcMarkWorkAvailable(nil) {
		65  		throw("GC work not flushed")
		66  	}
		67  	useCheckmark = false
		68  }
		69  
		70  // setCheckmark throws if marking object is a checkmarks violation,
		71  // and otherwise sets obj's checkmark. It returns true if obj was
		72  // already checkmarked.
		73  func setCheckmark(obj, base, off uintptr, mbits markBits) bool {
		74  	if !mbits.isMarked() {
		75  		printlock()
		76  		print("runtime: checkmarks found unexpected unmarked object obj=", hex(obj), "\n")
		77  		print("runtime: found obj at *(", hex(base), "+", hex(off), ")\n")
		78  
		79  		// Dump the source (base) object
		80  		gcDumpObject("base", base, off)
		81  
		82  		// Dump the object
		83  		gcDumpObject("obj", obj, ^uintptr(0))
		84  
		85  		getg().m.traceback = 2
		86  		throw("checkmark found unmarked object")
		87  	}
		88  
		89  	ai := arenaIndex(obj)
		90  	arena := mheap_.arenas[ai.l1()][ai.l2()]
		91  	arenaWord := (obj / heapArenaBytes / 8) % uintptr(len(arena.checkmarks))
		92  	mask := byte(1 << ((obj / heapArenaBytes) % 8))
		93  	bytep := &arena.checkmarks[arenaWord]
		94  
		95  	if atomic.Load8(bytep)&mask != 0 {
		96  		// Already checkmarked.
		97  		return true
		98  	}
		99  
	 100  	atomic.Or8(bytep, mask)
	 101  	return false
	 102  }
	 103
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